In the steel and aluminum milling industry, galvanized iron, galvanized steel and Galvalume® articles are generally treated with a chemical passivating rinse shortly after galvanization in order to prevent corrosion during storage and shipping. Chemical passivating rinses are known in the art and may contain chromium, phosphates, polymeric materials, surfactants and other additives designed to prevent oxidation and/or hydrolysis of the zinciferous metal surfaces of the article. To date the most effective anti-corrosion passivates have been those containing chromium. The majority of galvanized metal that is transported via oceangoing vessels is passivated with a chromium-containing composition as protection against potential exposure to salt water.
The chromium-containing passivates have the drawback of providing poor adhesion of subsequently applied organic coatings applied directly to the passivated surface. This problem has been recognized in the industry for years and no commercially viable solution has previously been found. The durability of the chromium-containing passivate on zinc surfaces increases the difficulty of removing the passivate when subsequent painting is desired. Thus, there is a need for a treatment for zinc-galvanized metals, such as galvanized iron, galvanized steel and Galvalume® articles, which have been chemically treated with a chromium-containing passivate, that improves adhesion of subsequently applied organic coatings, such as paint, lacquer and the like, that can be applied to the article without first removing all or substantially all of the passivate.
The excellent corrosion resistance and poor paint adherence features of chromium-containing passivates have caused some steel and aluminum mills to generate two product streams, one intended for later painting which is not treated with chromium-containing passivate and another, not intended to be painted, that is treated with chromium-containing passivate. The two product streams have resulted in duplicate inventories and other inefficiencies in the industry. Typical chromium-containing passivates are not easily detectable by the human eye, which has resulted in the shipment of the wrong product to customers. The industry has resorted to test procedures to determine whether a metal panel has been passivated using chromium. Shipping errors and testing to verify the absence of chromium-containing passivate are a cost to the manufacturer and customer. Thus, there is a need for a treatment for chemically passivated zinciferous metal surfaces, to improve adhesion of subsequently applied organic coatings, such as paint, lacquer and the like, which would enable the steel and aluminum mills to use a chromium-containing passivate for all galvanized products (intended to be painted or to remain unpainted), thus eliminating dual inventories and product testing of the passivated surface to determine whether a chromium containing passivate is present.
It is known to coat unpassivated or non-chrome passivated zinc surfaces of galvanized metals with aqueous coating solutions that are effective in providing corrosion resistant coatings. In addition to serving to prevent or inhibit corrosion, such coatings may also increase the adhesion properties of the surface to organic coatings such as paints, lacquers and the like, which may be subsequently applied to the metal. A general type of treatment composition for inhibiting corrosion and increasing adhesion of siccative coatings is disclosed in WO/85/05 131, in which the zinc surface after cleaning and rinsing, is coated with the composition, rinsed with water and lastly, rinsed with a chrome containing passivate. U.S. Pat. No. 5,344,504 to Deck et al. teaches compositions of: a fluorometallic acid, a transition metal salt, and zinc. In the process for applying these coatings a zinc surface, e.g. galvanized, is cleaned using an alkaline cleaner, rinsed with water, coated with the coating solution, rinsed with water, and then rinsed with an acidulated chrome solution. A siccative coating can thereafter be applied to the metal. This process has the drawback of requiring operation in a very narrow pH range of between 4 and 5. None of the aforementioned publications address the unique problem posed by the need to paint zinciferous metal surfaces that have already been chromium passivated. Thus there remains a need for a process of treating chromium passivated, zinciferous metal surfaces which provides increased resistance to corrosion and improved adherence of a subsequently applied organic coating. No commercially available products provide sufficient paint adhesion and corrosion protection to meet manufacturers' requirements.